Significant speedup by reusing a static NSDateFormatter instance.

Instantiating a NSDateFormatter instance is terribly slow, as documented a couple of places:

http://patzearfoss.com/2011/09/22/in-case-you-didnt-know-nsdateformatter-is-slooooow/
https://alpha.app.net/tewha/post/8055698
http://cocoaintheshell.com/2010/08/nsdateformatter-performances/

Holding on to a single static date formatter instance has neglible memory impact while increasing performance significantly.